1. Field of the Invention
The present invention generally relates to a surface acoustic wave device and a duplexer, and more particularly, to a surface acoustic wave device and a duplexer in which a piezoelectric substrate is joined on a supporting substrate.
2. Description of the Related Art
Recently, cellular phones, personal digital accesses and the like have been spread rapidly according to the progress of the mobile communication system. For example, a high-frequency band such as a frequency band from 800 MHz to 1.0 GHz and from 1.5 GHz to 2.0 GHz is used for cellular phones. A surface acoustic wave device such as a surface acoustic wave filter and a duplexer are used in this area.
A piezoelectric substrate used for surface acoustic wave devices such as a LiTaO3 (lithium tantalate) LiNbO3 (lithium niobate) substrate has an advantage that a large electromechanical coupling coefficient is available, and a disadvantage that the resonance (or anti-resonance) frequency thereof depends on temperature greatly. In contrast, crystal has a resonance (or anti-resonance) frequency that does not depend on temperature greatly, while having a small electromechanical coupling coefficient.
Japanese Patent Application Publication Nos. 2004-186868, 2005-252550 and 2004-343359 (hereinafter referred to as Documents 1, 2 and 3, respectively) disclose a surface acoustic wave device in which a LiTaO3 piezoelectric substrate is joined on a sapphire substrate and an electrode pattern such as IDT (InterDigital Transducer) is provided on the piezoelectric substrate.
In Documents 1 and 2, the LiTaO3 piezoelectric substrate (an LT substrate) has a normal line direction on the surface thereof in a direction that is rotated 42° from a Y axis to a Z axis direction about an X axis in which a surface acoustic wave is propagated (this is called a 42° cut substrate). The 42° cut substrate is mainly used for the surface acoustic wave device including the LT substrate, since the propagation loss thereof is small. However, in the surface acoustic wave device described in Documents 1 through 3, ripples are generated in a resonance characteristic of resonator or a pass band of filter.